What TheNightSlasher said about the general strategy with volus.
For the weapons :
- Adept : he has Stasis, so take advantage of it by using a good headshot weapon. Arc Pistol is my favorite on him, but Paladin is good too if you're not comfortable with charged weapons. For the ammo, you need something with a bonus against armor. AP, drill and incendiary are the best choices (warp rounds are wasted on him since he can't prime armor).
- Engineer : I have varied a lot the weapons used on him, but now I always take the Wraith. With the debuff from his powers and enough gear, you can two-shots kill all shielded mooks like Marauders, Centurions, etc. with this gun. And it's light, so the cooldowns of the mines stay decent. Use AP or drill rounds (since he has wallhax capabilities with Recon Mine).
- Sentinel : definitively PPR. Given the very low damage of his powers, the volus merc needs a gun with the highest possible dps. And the staggering / distracting abilities of his powers allow you to deal easily with the charge-up mechanism of this gun and to get easy headshots. Use AP rounds, eventually incendiary or disruptor to get fire explosions or tech bursts with the Drone (rank 4b).
- Vanguard : Acolyte + Hurricane with incendiary rounds. Given his squishyness, you want your ennemy dead after your charge, hence the incendiary Acolyte. So shoot, charge, roll, use your cloak, take cover or run to an other ennemy, but try to always stay very mobile after a charge, you can be downed very fast with him.
About the builds, no fitness makes the most sense since you have Shield Boost. However, in your process to become better with the volus, you can start with some points in fitness, the time to get comfortable with their squishyness. For that, you can sacrifice rank 6 of Proxy Mine, and ranks 4 to 6 on the Decoy and the Orbs, these powers already do a good job with 3 ranks in them.